BACKGROUND: Subjective effects of cocaine are mediated primarily by dopamine (DA) transporter (DAT) blockade. The present study assessed the hypothesis that different DAT conformational equilibria regulate differences in cocaine-like subjective effects and extracellular DA induced by diverse DA-uptake inhibitors (DUIs). METHODS: The relationship between cocaine-like subjective effects and stimulation of mesolimbic DA levels by standard DUIs (cocaine, methylphenidate, WIN35,428) and atypical DUIs (benztropine analogs: AHN1-055, AHN2-005, JHW007) was investigated using cocaine discrimination and DA microdialysis procedures in rats. RESULTS: All drugs stimulated DA levels with different maxima and time courses. Standard DUIs, which preferentially bind outward-facing DAT conformations, fully substituted for cocaine, consistently producing cocaine-like subjective effects at DA levels of 100-125% over basal values, regardless of dose or pretreatment time. The atypical DUIs, with DAT binding minimally affected by DAT conformation, produced inconsistent cocaine-like subjective effects. Full effects were obtained, if at all, only at a few doses and pretreatment times and at DA levels 600-700% greater than basal values. Importantly, the linear, time-independent, relationship between cocaine-like subjective effects and DA stimulation obtained with standard DUIs was not obtained with the atypical DUIs. CONCLUSIONS: These results suggest a time-related desensitization process underlying the reduced cocaine subjective effects of atypical DUIs that may be differentially induced by the binding modalities identified using molecular approaches. Since the DAT is the target of several drugs for treating neuropsychiatric disorders, such as attention-deficit/hyperactivity disorder, these results help to identify safe and effective medications with minimal cocaine-like subjective effects that contribute to abuse liability.

RATIONALE: The rewarding effects of alcohol have been attributed to interactions between opioid and dopaminergic system within the mesolimbic reward pathway. We have previously shown that ablation of beta-arrestin 2 (Arrb2), a crucial regulator of mu-opioid receptor function, attenuates alcohol-induced hyperlocomotion and c-fos activation in the nucleus accumbens. OBJECTIVES: Here, we further investigated the role of Arrb2 in modulating alcohol-induced dopamine (DA) release and conditioned place preference (CPP). We also assessed the functional importance of Arrb2 for mu-opioid receptor surface expression and signaling following an acute alcohol challenge. METHODS: Alcohol-evoked (0.375, 0.75, and 1.5 g/kg intraperitoneally) DA release was measured by in vivo microdialysis in the shell of nucleus accumbens. Reward was assessed by the CPP paradigm. Receptor function was assessed by mu-receptor binding and [(35)S]GTP-gamma-S autoradiography. RESULTS: In Arrb2 knockout mice accumbal DA levels reach maximum response at a lower dose compared to wild-type (wt) animals. In line with these results, Arrb2 knockout mice display increased CPP for alcohol as compared to wt mice. Finally, Arrb2 mutant mice display increased mu-opioid receptor signaling in the ventral and dorsal striatum and amygdala in response to a low dose of alcohol, indicating impaired desensitization mechanisms in these mice. CONCLUSIONS: Our results show that Arrb2 modulates the response to low doses of alcohol on various levels including mu-opioid receptor signaling, DA release, and reward. They also reveal a clear dissociation between the effects of Arrb2 on psychomotor and reward behaviors.

In the reward circuitry of the brain, alpha-7-nicotinic acetylcholine receptors (alpha7nAChRs) modulate effects of Delta(9)-tetrahydrocannabinol (THC), marijuana's main psychoactive ingredient. Kynurenic acid (KYNA) is an endogenous negative allosteric modulator of alpha7nAChRs. Here we report that the kynurenine 3-monooxygenase (KMO) inhibitor Ro 61-8048 increases brain KYNA levels and attenuates cannabinoid-induced increases in extracellular dopamine in reward-related brain areas. In the self-administration model of drug abuse, Ro 61-8048 reduced the rewarding effects of THC and the synthetic cannabinoid WIN 55,212-2 in squirrel monkeys and rats, respectively, and it also prevented relapse to drug-seeking induced by reexposure to cannabinoids or cannabinoid-associated cues. The effects of enhancing endogenous KYNA levels with Ro 61-8048 were prevented by positive allosteric modulators of alpha7nAChRs. Despite a clear need, there are no medications approved for treatment of marijuana dependence. Modulation of KYNA offers a pharmacological strategy for achieving abstinence from marijuana and preventing relapse.

@article{Mereu2013,
title = {The neurobiology of modafinil as an enhancer of cognitive performance and a potential treatment for substance use disorders.},
author = {Maddalena Mereu and Antonello Bonci and Amy Hauck Newman and Gianluigi Tanda},
url = {https://www.ncbi.nlm.nih.gov/pubmed/23934211},
doi = {10.1007/s00213-013-3232-4},
issn = {1432-2072 (Electronic); 0033-3158 (Linking)},
year = {2013},
date = {2013-10-01},
journal = {Psychopharmacology (Berl)},
volume = {229},
number = {3},
pages = {415--434},
address = {Molecular Targets and Medication Discovery Branch, National Institute on Drug Abuse, Intramural Research Program, National Institutes of Health, DHHS, 251 Bayview Boulevard, NIDA Suite 200, Baltimore, MD, USA.},
abstract = {RATIONALE AND OBJECTIVES: Modafinil (MOD) and its R-enantiomer (R-MOD) are approved medications for narcolepsy and other sleep disorders. They have also been used, off-label, as cognitive enhancers in populations of patients with mental disorders, including substance abusers that demonstrate impaired cognitive function. A debated nonmedical use of MOD in healthy individuals to improve intellectual performance is raising questions about its potential abuse liability in this population. RESULTS AND CONCLUSIONS: MOD has low micromolar affinity for the dopamine transporter (DAT). Inhibition of dopamine (DA) reuptake via the DAT explains the enhancement of DA levels in several brain areas, an effect shared with psychostimulants like cocaine, methylphenidate, and the amphetamines. However, its neurochemical effects and anatomical pattern of brain area activation differ from typical psychostimulants and are consistent with its beneficial effects on cognitive performance processes such as attention, learning, and memory. At variance with typical psychostimulants, MOD shows very low, if any, abuse liability, in spite of its use as a cognitive enhancer by otherwise healthy individuals. Finally, recent clinical studies have focused on the potential use of MOD as a medication for treatment of drug abuse, but have not shown consistent outcomes. However, positive trends in several result measures suggest that medications that improve cognitive function, like MOD or R-MOD, may be beneficial for the treatment of substance use disorders in certain patient populations.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

RATIONALE AND OBJECTIVES: Modafinil (MOD) and its R-enantiomer (R-MOD) are approved medications for narcolepsy and other sleep disorders. They have also been used, off-label, as cognitive enhancers in populations of patients with mental disorders, including substance abusers that demonstrate impaired cognitive function. A debated nonmedical use of MOD in healthy individuals to improve intellectual performance is raising questions about its potential abuse liability in this population. RESULTS AND CONCLUSIONS: MOD has low micromolar affinity for the dopamine transporter (DAT). Inhibition of dopamine (DA) reuptake via the DAT explains the enhancement of DA levels in several brain areas, an effect shared with psychostimulants like cocaine, methylphenidate, and the amphetamines. However, its neurochemical effects and anatomical pattern of brain area activation differ from typical psychostimulants and are consistent with its beneficial effects on cognitive performance processes such as attention, learning, and memory. At variance with typical psychostimulants, MOD shows very low, if any, abuse liability, in spite of its use as a cognitive enhancer by otherwise healthy individuals. Finally, recent clinical studies have focused on the potential use of MOD as a medication for treatment of drug abuse, but have not shown consistent outcomes. However, positive trends in several result measures suggest that medications that improve cognitive function, like MOD or R-MOD, may be beneficial for the treatment of substance use disorders in certain patient populations.

A previous study showed that cocaine self-administration induced dopamine-independent reinforcing effects of sigma agonists mediated by their selective actions at sigma1 receptors (sigma1Rs), which are intracellularly mobile chaperone proteins implicated in abuse-related effects of stimulants. The present study assessed whether the induction was specific to self-administration of cocaine. Rats were trained to self-administer the dopamine releaser, d-methamphetamine (0.01-0.32 mg/kg per injection), the mu-opioid receptor agonist, heroin (0.001-0.032 mg/kg per injection), and the noncompetitive N-methyl-d-aspartate receptor/channel antagonist ketamine (0.032-1.0 mg/kg per injection). As with cocaine, self-administration of d-methamphetamine induced reinforcing effects of the selective sigma1R agonists PRE-084 [2-(4-morpholinethyl)1-phenylcyclohexanecarboxylate hydrochloride] and (+)-pentazocine (0.032-1.0 mg/kg per injection, each). In contrast, neither self-administration of heroin nor ketamine induced PRE-084 or (+)-pentazocine (0.032-10 mg/kg per injection, each) self-administration. Although the sigma1R agonists did not maintain responding in subjects with histories of heroin or ketamine self-administration, substitution for those drugs was obtained with appropriate agonists (e.g., remifentanil, 0.1-3.2 microg/kg per injection, for heroin and (5S,10R)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine ((+)-MK 801; dizocilpine), 0.32-10.0 microg/kg per injection, for ketamine). The sigmaR antagonist N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine dihydrobromide (BD 1008; 1.0-10 mg/kg) dose-dependently blocked PRE-084 self-administration but was inactive against d-methamphetamine, heroin, and ketamine. In contrast, PRE-084 self-administration was affected neither by the dopamine receptor antagonist (+)-butaclamol (10-100 mug/kg) nor by the opioid antagonist (-)-naltrexone (1.0-10 mg/kg), whereas these antagonists were active against d-methamphetamine and heroin self-administration, respectively. The results indicate that experience specifically with indirect-acting dopamine agonists induces reinforcing effects of previously inactive sigma1R agonists. It is further suggested that induced sigma1R reinforcing mechanisms may play an essential role in treatment-resistant stimulant abuse, suggesting new approaches for the development of effective medications for its treatment.

@article{Tanda2013,
title = {Relations between stimulation of mesolimbic dopamine and place conditioning in rats produced by cocaine or drugs that are tolerant to dopamine transporter conformational change.},
author = {Gianluigi Tanda and Su Min Li and Maddalena Mereu and Alexandra M Thomas and Aaron L Ebbs and Lauren E Chun and Valeria Tronci and Jennifer L Green and Mu-Fa Zou and Theresa A Kopajtic and Amy Hauck Newman and Jonathan L Katz},
url = {https://www.ncbi.nlm.nih.gov/pubmed/23612854},
doi = {10.1007/s00213-013-3109-6},
issn = {1432-2072 (Electronic); 0033-3158 (Linking)},
year = {2013},
date = {2013-09-01},
journal = {Psychopharmacology (Berl)},
volume = {229},
number = {2},
pages = {307--321},
address = {Psychobiology Section, National Institute on Drug Abuse, Intramural Research Program, National Institutes of Health, 251 Bayview Blvd., NIDA suite 200, Baltimore, MD 21224, USA.},
abstract = {RATIONALE: Dopamine transporter (DAT) conformation plays a role in the effectiveness of cocaine-like and other DAT inhibitors. Cocaine-like stimulants are intolerant to DAT conformation changes having decreased potency in cells transfected with DAT constructs that face the cytosol compared to wild-type DAT. In contrast, analogs of benztropine (BZT) are among compounds that are less affected by DAT conformational change. METHODS: We compared the displacement of radioligand binding to various mammalian CNS sites, acute stimulation of accumbens shell dopamine levels, and place conditioning in rats among cocaine and four BZT analogs with Cl substitutions on the diphenyl-ether system including two with carboalkoxy substitutions at the 2-position of the tropane ring. RESULTS: Binding assays confirmed high-affinity and selectivity for the DAT with the BZT analogs which also produced significant stimulation of mesolimbic dopamine efflux. Because BZT analogs produced temporal patterns of extracellular dopamine levels different from those by cocaine (3-10 mg/kg, i.p.), the place conditioning produced by BZT analogs and cocaine was compared at doses and times at which both the increase in dopamine levels and rates of increase were similar to those produced by an effective dose of cocaine. Despite this equilibration, none of the BZT analogs tested produced significant place conditioning. CONCLUSIONS: The present results extend previous findings suggesting that cocaine-like actions are dependent on a binding equilibrium that favors the outward conformational state of the DAT. In contrast, BZT analogs with reduced dependence on DAT conformation have reduced cocaine-like behavioral effects and may prove useful in development of medications for stimulant abuse.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

RATIONALE: Dopamine transporter (DAT) conformation plays a role in the effectiveness of cocaine-like and other DAT inhibitors. Cocaine-like stimulants are intolerant to DAT conformation changes having decreased potency in cells transfected with DAT constructs that face the cytosol compared to wild-type DAT. In contrast, analogs of benztropine (BZT) are among compounds that are less affected by DAT conformational change. METHODS: We compared the displacement of radioligand binding to various mammalian CNS sites, acute stimulation of accumbens shell dopamine levels, and place conditioning in rats among cocaine and four BZT analogs with Cl substitutions on the diphenyl-ether system including two with carboalkoxy substitutions at the 2-position of the tropane ring. RESULTS: Binding assays confirmed high-affinity and selectivity for the DAT with the BZT analogs which also produced significant stimulation of mesolimbic dopamine efflux. Because BZT analogs produced temporal patterns of extracellular dopamine levels different from those by cocaine (3-10 mg/kg, i.p.), the place conditioning produced by BZT analogs and cocaine was compared at doses and times at which both the increase in dopamine levels and rates of increase were similar to those produced by an effective dose of cocaine. Despite this equilibration, none of the BZT analogs tested produced significant place conditioning. CONCLUSIONS: The present results extend previous findings suggesting that cocaine-like actions are dependent on a binding equilibrium that favors the outward conformational state of the DAT. In contrast, BZT analogs with reduced dependence on DAT conformation have reduced cocaine-like behavioral effects and may prove useful in development of medications for stimulant abuse.

@article{Hiranita2013,
title = {Self-administration of cocaine induces dopamine-independent self-administration of sigma agonists.},
author = {Takato Hiranita and Maddalena Mereu and Paul L Soto and Gianluigi Tanda and Jonathan L Katz},
url = {https://www.ncbi.nlm.nih.gov/pubmed/23187725},
doi = {10.1038/npp.2012.224},
issn = {1740-634X (Electronic); 0893-133X (Linking)},
year = {2013},
date = {2013-03-01},
journal = {Neuropsychopharmacology},
volume = {38},
number = {4},
pages = {605--615},
address = {Psychobiology Section, Molecular Targets and Medications Discovery Branch, Intramural Research Program, Department of Health and Human Services, National Institute on Drug Abuse, National Institutes of Health, Baltimore, MD, USA.},
abstract = {Sigma(1) receptors (sigma(1)Rs) are intracellularly mobile chaperone proteins implicated in several disease processes, as well as psychiatric disorders and substance abuse. Here we report that although selective sigma(1)R agonists (PRE-084, (+)-pentazocine) lacked reinforcing effects in drug-naive rats, over the course of 28 experimental sessions, which was more than sufficient for acquisition of cocaine self-administration, responding was not maintained by either sigma(1)R agonist. In contrast, after subjects self-administered cocaine sigma(1)R agonists were readily self-administered. The induced reinforcing effects were long lasting; a response for which subjects had no history of reinforcement was newly conditioned with both sigma(1)R agonists, extinguished when injections were discontinued, and reconditioned when sigma(1)R agonists again followed responses. Experience with food reinforcement was ineffective as an inducer of sigma(1)R agonist reinforcement. Although a variety of dopamine receptor antagonists blocked cocaine self-administration, consistent with its dopaminergic mechanism, PRE-084 self-administration was entirely insensitive to these drugs. Conversely, the sigmaR antagonist, BD1063, blocked PRE-084 self-administration but was inactive against cocaine. In microdialysis studies i.v. PRE-084 did not significantly stimulate dopamine at doses that were self-administered in rats either with or without a cocaine self-administration experience. The results indicate that cocaine experience induces reinforcing effects of previously inactive sigma(1)R agonists, and that the mechanism underlying these reinforcing effects is dopamine independent. It is further suggested that induced sigma(1)R mechanisms may have an essential role in treatment-resistant stimulant abuse, suggesting new approaches for the development of effective medications for stimulant abuse.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

Sigma(1) receptors (sigma(1)Rs) are intracellularly mobile chaperone proteins implicated in several disease processes, as well as psychiatric disorders and substance abuse. Here we report that although selective sigma(1)R agonists (PRE-084, (+)-pentazocine) lacked reinforcing effects in drug-naive rats, over the course of 28 experimental sessions, which was more than sufficient for acquisition of cocaine self-administration, responding was not maintained by either sigma(1)R agonist. In contrast, after subjects self-administered cocaine sigma(1)R agonists were readily self-administered. The induced reinforcing effects were long lasting; a response for which subjects had no history of reinforcement was newly conditioned with both sigma(1)R agonists, extinguished when injections were discontinued, and reconditioned when sigma(1)R agonists again followed responses. Experience with food reinforcement was ineffective as an inducer of sigma(1)R agonist reinforcement. Although a variety of dopamine receptor antagonists blocked cocaine self-administration, consistent with its dopaminergic mechanism, PRE-084 self-administration was entirely insensitive to these drugs. Conversely, the sigmaR antagonist, BD1063, blocked PRE-084 self-administration but was inactive against cocaine. In microdialysis studies i.v. PRE-084 did not significantly stimulate dopamine at doses that were self-administered in rats either with or without a cocaine self-administration experience. The results indicate that cocaine experience induces reinforcing effects of previously inactive sigma(1)R agonists, and that the mechanism underlying these reinforcing effects is dopamine independent. It is further suggested that induced sigma(1)R mechanisms may have an essential role in treatment-resistant stimulant abuse, suggesting new approaches for the development of effective medications for stimulant abuse.

BACKGROUND: (+/-)-Modafinil has piqued interest as a treatment for attention-deficit/hyperactivity disorder and stimulant dependence. The R-enantiomer of modafinil might have unique pharmacological properties that should be further investigated. METHODS: (+/-)-Modafinil and its R-(-)- and S-(+)-enantiomers were synthesized and tested for inhibition of [(3)H] dopamine (DA) uptake and [(3)H]WIN 35428 binding in human dopamine transporter (DAT) wild-type and mutants with altered conformational equilibria. Data were compared with cocaine and the atypical DA uptake inhibitor, JHW 007. R- and S-modafinil were also evaluated in microdialysis studies in the mouse nucleus accumbens shell and in a cocaine discrimination procedure. RESULTS: (+/-)-, R-, and S-modafinil bind to the DAT and inhibit DA uptake less potently than cocaine, with R-modafinil having approximately threefold higher affinity than its S-enantiomer. Molecular docking studies revealed subtle differences in binding modes for the enantiomers. R-modafinil was significantly less potent in the DAT Y156F mutant compared with wild-type DAT, whereas S-modafinil was affected less. Studies with the Y335A DAT mutant showed that the R- and S-enantiomers tolerated the inward-facing conformation better than cocaine, which was further supported by [2-(trimethylammonium)ethyl]-methanethiosulfonate reactivity on the DAT E2C I159C. Microdialysis studies demonstrated that both R- and S-modafinil produced increases in extracellular DA concentrations in the nucleus accumbens shell less efficaciously than cocaine and with a longer duration of action. Both enantiomers fully substituted in mice trained to discriminate cocaine from saline. CONCLUSIONS: R-modafinil displays an in vitro profile different from cocaine. Future trials with R-modafinil as a substitute therapy with the potential benefit of cognitive enhancement for psychostimulant addiction are warranted.

@article{Chiara1999,
title = {Reciprocal changes in prefrontal and limbic dopamine responsiveness to aversive and rewarding stimuli after chronic mild stress: implications for the psychobiology of depression.},
author = {G Di Chiara and P Loddo and G Tanda},
url = {https://www.ncbi.nlm.nih.gov/pubmed/10624543},
issn = {0006-3223 (Print); 0006-3223 (Linking)},
year = {1999},
date = {1999-12-15},
journal = {Biol Psychiatry},
volume = {46},
number = {12},
pages = {1624--1633},
address = {Department of Toxicology, University of Cagliari, Italy.},
abstract = {BACKGROUND: Chronic mild stress (CMS) has been reported to induce behavioral abnormalities that model human depression. To investigate the role in depression of phasic dopamine transmission in cortical and limbic areas, we studied the effect of CMS on the responsiveness of dopamine (DA) transmission to aversive and rewarding stimuli in rats by microdialysis of the nucleus accumbens (NAc) shell and of the medial prefrontal cortex (PFCX). METHODS: Rats were subjected for 30 days to CMS and administered two trials of tail pinch as aversive stimulus and two feeding sessions of a highly palatable food as rewarding stimulus. Concentric microdialysis probes were implanted in the NAc shell and in the medial PFCX. RESULTS: In unstressed rats, DA decreased in the NAc and increased in the PFCX on the first tail-pinch trial; on the 1st feeding trial, DA increased in the NAc and to a larger extent in the PFCX. In the second tail-pinch trial or feeding trial, these responses were maintained in the PFCX but underwent habituation in the NAc. CMS did not affect basal dialysate DA in the NAc or in the PFCX but influenced the responsiveness of Da transmission to tail pinches and to feeding in a reciprocal manner. Thus, in the tail-pinch trial, CMS reversed the inhibitory response of NAc DA transmission into a stimulatory one and potentiated the stimulatory response in the PFCX. By contrast, in the feeding trial, CMS blunted the stimulatory response of DA transmission in the NAc in the first trial and in the PFCX in the second trial. CONCLUSIONS: CMS reciprocally affected DA responsiveness to motivational stimuli, facilitating or inducing a stimulatory DA response to aversive stimuli but blunting stimulatory responses to rewarding stimuli. Given the postulated role of phasic DA responsiveness in the NAc shell for learning and of DA transmission in the PFCX for expression of motivation, we hypothesize that depression is the result of defective learning and expression of aversive and appetitive motivation.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

BACKGROUND: Chronic mild stress (CMS) has been reported to induce behavioral abnormalities that model human depression. To investigate the role in depression of phasic dopamine transmission in cortical and limbic areas, we studied the effect of CMS on the responsiveness of dopamine (DA) transmission to aversive and rewarding stimuli in rats by microdialysis of the nucleus accumbens (NAc) shell and of the medial prefrontal cortex (PFCX). METHODS: Rats were subjected for 30 days to CMS and administered two trials of tail pinch as aversive stimulus and two feeding sessions of a highly palatable food as rewarding stimulus. Concentric microdialysis probes were implanted in the NAc shell and in the medial PFCX. RESULTS: In unstressed rats, DA decreased in the NAc and increased in the PFCX on the first tail-pinch trial; on the 1st feeding trial, DA increased in the NAc and to a larger extent in the PFCX. In the second tail-pinch trial or feeding trial, these responses were maintained in the PFCX but underwent habituation in the NAc. CMS did not affect basal dialysate DA in the NAc or in the PFCX but influenced the responsiveness of Da transmission to tail pinches and to feeding in a reciprocal manner. Thus, in the tail-pinch trial, CMS reversed the inhibitory response of NAc DA transmission into a stimulatory one and potentiated the stimulatory response in the PFCX. By contrast, in the feeding trial, CMS blunted the stimulatory response of DA transmission in the NAc in the first trial and in the PFCX in the second trial. CONCLUSIONS: CMS reciprocally affected DA responsiveness to motivational stimuli, facilitating or inducing a stimulatory DA response to aversive stimuli but blunting stimulatory responses to rewarding stimuli. Given the postulated role of phasic DA responsiveness in the NAc shell for learning and of DA transmission in the PFCX for expression of motivation, we hypothesize that depression is the result of defective learning and expression of aversive and appetitive motivation.